In the advanced lithography process for the fabrication of semiconductor devices, a light source of shorter wavelength is used for exposure. A subsequent transition to lithography using extreme ultraviolet (EUV) is regarded promising.
Since the EUV lithography uses a reflecting optical system, the lithography accuracy can be adversely affected even by a slight thermal expansion of each member (e.g., substrate) in the lithographic optical system induced by the heat that has reached there. Accordingly, members like reflecting mirrors, masks, and stages must be made of low expansion materials. Titania-doped quartz glass is known as a typical low expansion material. The addition of a certain amount of titania makes it possible to minimize the thermal expansion of quartz glass.
The EUV lithography members, especially photomask-forming substrates are required to be fully flat. Specifically, photomask substrates must have a flatness of up to 50 nm in the practical application, and an even high flatness of up to 30 nm for the fabrication of finer patterns, within a central region of 142 mm×142 mm squares.
However, it is difficult to manufacture substrates having a high flatness from titania-doped quartz glass when the glass has a non-uniform titania concentration. When a glass substrate having a non-uniform titania concentration is polished, the substrate surface becomes irregular due to varying reactivity with the polishing slurry and differential polishing speed. In this regard, JP-A 2004-315351, JP-A 2005-104820, and JP-A 2005-022954, for example, disclose titania-doped quartz glass having a narrow titania concentration distribution useful as EUV lithographic members.
JP-A 2010-013335 describes that the refractive index distribution of titania-doped quartz glass is determined by taking into account the polishing mechanism so that high-flatness substrates may be readily manufactured therefrom.
During manufacture of titania-doped quartz glass, zones having a non-uniform titania concentration, known as striae, may be formed in a direction perpendicular to the growth direction of titania-doped quartz glass, due to temperature variations at the growth face, variations of the reactant gas composition, and other variations. Striae are variations of titania concentration at intervals of several microns to several millimeters, and structurally strained sites are present within the stria. Since strained sites within titania-doped quartz glass are structurally unstable, abrasion selectively occurs thereat during polishing, leading to aggravated flatness.
Based on the discovery that striae-strained sites are converted into numerical values of stress, JP-A 2010-135732 proposes the stress level permissible as EUV lithographic members and the method of reducing the stress.